Referring to FIGS. 1 and 2, a secondary fuel nozzle according to the prior art comprises a secondary fuel nozzle body 2 and fuel piping 4 welded to the nozzle body 2. The fuel piping 4 comprises a plurality of pipes and each pipe includes a flange 6. As shown in FIG. 2, the fuel piping 4 is connected to flexible hoses 10. The flexible hoses 10 each include a flange 8 that is connected to the flange 6 of a respective fuel pipe 4.
The welded fuel piping 4 is susceptible to high cycle fatigue cracks due to vibration. The flange 6 to flange 8 connection presents a cantilevered mass that has a given natural frequency. Gas turbine rotor vibrations cause high amplitude response of the piping 4 resulting in cracks in the welds between the piping 4 and the fuel nozzle body 2. There is also a potential for high cycle fatigue initiation during unit startup.
Prior attempts to solve these problems included increasing robustness of welds adjoining the piping to the nozzle, changing from slip-on to weld neck flanges, adding weld adapters to increase strength of life limiting weld, and in extreme cases adding welded bracing fixtures to stiffen the piping.